A protein termed T (tumor) antigen appears in the nuclei of cells which have been either lytically infected or transformed by tumor viruses such as SV40 and polyoma virus. We have recently shown that SV40 T antigen is a DNA binding protein which has high affinity for SV40 superhelical double stranded DNA. We propose to develop a millipore DNA binding assay for T antigen, to purify T antigen, to characterize the purified protein and hopefully to study its function. We plan to work first with normal T antigen from SV40 and with T antigens from temperature sensitive SV40 transformed cells such as the ts a mutant which does not form T antigen at the non permissive temperature. Secondly we should like to extend these studies to T antigen from polyoma virus and its appropriate temperature sensitive mutants. We also plan to use a new surface labeling agent, chloroperoxidase and Cl36 to examine cell surface changes between virally transformed and normal cells.